The present invention relates generally to semiconductor devices and, more particularly, to an apparatus and method for improving sensing margin of electrically programmable fuse type devices in semiconductor devices.
Electrically programmable fuses (eFUSEs) are widely used to implement memory redundancy functionality in dynamic random access memory (DRAM), static random access memory (SRAM) and embedded memory devices. Programmable fuses can also be utilized in applications such as electronic chip identification, product feature personalization, and thermal diode calibration, to name a few.
In a redundancy application, for example, chips returned from fabrication are usually tested and a certain percentage of them are usually found to be bad (e.g., from random contingencies that may occur in the fabrication process). The percentage of good, usable chips is commonly referred to as the “yield.” Accordingly, redundancy may enable programming a chip at the testing phase so that flawed portions of the chip are not used in favor of unflawed redundant portions, thereby increasing the yield and decreasing the percentage of chips that must be thrown away as unusable. Many of the applications for electronic fuses combine the electronic fuse with the use of a memory in some way, for example, as part of a memory cell.
Additionally, in electronic chip identification or part number identification, an eFUSE may be used to “program” or write information into a non-volatile memory, which may be comprised of individual memory cells, and the information can then be read from the memory as a unique chip identifier or part number identifier. Furthermore, for thermal diode calibration, a non-volatile memory programmed using electronic fuses can be used to hold a test temperature and the thermal diode voltage read at the test temperature.
Regardless of the specific application environment, an eFUSE is typically programmed by passing a sufficient current through the structure such that its resistance is significantly altered from its initially fabricated state. In order to determine whether a particular fuse has been programmed or not, a sense circuit may be used to detect one of two possible “states” of the fuse. More specifically, the sense circuit holds one of two latched values therein, which is driven by a comparison between an applied voltage (by the sense circuit) across the fuse and a reference voltage generated within the sense circuit. The reference voltage is designed to be between a fuse voltage corresponding to the programmed state and a fuse voltage corresponding to the unprogrammed state.
However, as is typically the case with semiconductor circuit devices, the devices that make up the sense circuit are susceptible to parameter changes (e.g., voltage threshold changes in the case of field effect transistors) during the lifetime of the product in the field. Furthermore, as devices continually move to smaller geometries and lower operating voltages, the structural differences between programmed and unprogrammed fuses become less, thus making it more difficult to sense the state of the fuse. In order to compensate for such variability, sense circuits have been designed using statistical simulations of the expected operating conditions, including power supply voltage, temperature, manufacturing process variations, and fuse resistance. The circuit devices are then adjusted as needed to achieve a sufficient sense margin (VFUSE−VREF) so as to tolerate the simulated variability.
Alternatively and/or additionally, fuse resistances are measured at the time of fuse programming (e.g., at wafer final test (WFT)) to assure that sufficient margin exists to successfully sense both intact and programmed fuses during the lifetime of the product. Unfortunately, the time required for implementing comprehensive fuse resistance measurement can be prohibitively long at WFT, particularly in chip designs employing thousands of fuses. It is desirable to enable a different sense margin at WFT with a digital circuit to quickly detect weak fuse cells. Accordingly, there remains a need for improving the sensing margin of electrically programmable fuses in semiconductor devices.